Aerobic biological treatments of effluents generally consist in contacting these effluents with a biomass (microorganisms), which degrades the pollution present in the effluents by converting the organic molecules to inorganic molecules; this step is commonly called the aeration of the biological tanks. The application of such treatments causes a progressive increase in the quantity of biomass and creates a need to dispose of the excess biomass commonly called “excess sludge”. Various solutions have been proposed for dealing with the steadily growing quantity of this excess biological sludge and for its disposal.
A first family of methods consists in withdrawing this excess sludge after the biological treatment and either finding a suitable outlet for it or treating it in a specific degradation step. It can thus be used as a fertilizer in agriculture (spreading). However, compliance with environmental standards and the potential presence of micropollutants or heavy metals in the sludge have led to a reduction of this use. Another solution is to withdraw this sludge and incinerate it; this demands transporting it to an incinerator, incurring a cost. Moreover, difficulties in installing new incinerators are hindering the development of this solution. Another solution is to carry out wet oxidation of the excess sludge: this makes the sludge inorganic.
A second family of methods consists in reducing the production of sludge during the biological treatment. These solutions consist in using means serving to reduce sludge production during the biological process of removing pollution from water. These solutions consist in carrying out a partial lysis of the sludge, that is the destruction of part of the microorganisms making up the sludge by making them partially soluble. The products of this lysis, which contain at least partially soluble organic compounds, can then be sent to the head end of the effluent treatment to undergo biological treatment, during which the microorganisms will treat the lysis products. A first known lysis technique consists in applying mechanical action to the sludge from the biological treatment tank which bursts a portion of the cells of the microorganisms making up the excess sludge. This may involve mechanical grinding, compression/expansion, sonochemistry, etc. This technique is generally simple to apply but presents the drawback of only slightly reducing the production of excess sludge. Moreover, the energy cost is high. A second lysis technique is basic or acid attack using chemical agents possibly combined with a rise in temperature, but this technique demands readjustment of the pH of the solution obtained before its reinjection into the aeration tank. The drawback of this solution is that it increases the salinity of the hydrolyzed sludge, which can cause malfunctions in the biological treatment step. A third lysis technique is based on the action of oxidizing agents such as ozone, air, hydrogen peroxide and pressurized oxygen. The drawback of air, hydrogen peroxide and oxygen is that they are not efficient enough alone: they must be combined with heating and/or a catalyst, thereby also increasing the cost of these techniques. As to ozone, its use requires the installation of a particular device. In fact, in its use for reducing the volume of excess sludge, ozone injection is dissociated from the aeration step of the aeration tanks. The ozone-containing gas is injected into a reactor separate from the aeration tanks. This is a drawback, because the installation is costly and its application on existing units is complicated.
Document U.S. Pat. No. 5,573,670 mentions the possibility of injecting an ozone-containing gas with a very low ozone concentration (0.01 to 0.16% by weight of O3 with respect to O2) into an aeration tank of a biological treatment unit for aqueous effluents, for the sole purpose of preventing the formation of filamentous bacteria and of significantly reducing the Total Organic Carbon (TOC). No influence of this direct injection of low ozone gas on the proportion of excess sludge has been demonstrated.